How to authenticate against Azure Key Vault both in Azure and local development environment?

Time needed: 15 minutes

How to implement Azure Key Vault for local development & Managed Identity?

1. Add your dependencies

   Add these NuGet packages:
   Microsoft.Azure.Services.AppAuthentication
Microsoft.Extensions.Configuration.AzureKeyVault
   
   Versions (for me, anyway) were 1.6.2 and 2.1.1, respectively. These should work for at least .NET Core 3.1 (and probably newer).

2. Implement authentication

   Something like this should work:
   
   // This works in DEV environments (as long as you've run "az login" first) and in Azure as long as Managed Identity has been configured
var azureServiceTokenProvider = new AzureServiceTokenProvider();
var keyVaultClient = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(azureServiceTokenProvider.KeyVaultTokenCallback));
   
   And the using-statements should be somewhat like this:
   
   using Microsoft.Azure.KeyVault;
using Microsoft.Azure.Services.AppAuthentication;
   
   And after this, something like this should work:
   var secretBundle = await keyVaultClient.GetSecretAsync($"{Environment.GetEnvironmentVariable("KeyVaultName")}/secrets/{keyName}").ConfigureAwait(false);
var secret = secretBundle.Value;
3. (OPTION 1) Sign in to Visual Studio using the credentials that can access the Key Vault

   In case you CAN log in to Visual Studio with just the account that is able to connect to Azure Key Vault, that’s probably the easiest thing to do. That (cached) identity should be used as-is by the Azure Key Vault client.
   
   However, if you need to use multiple accounts in your Visual Studio, this won’t work. Well, in my experience, usually it won’t.
   
   But no worries, as there’s another way to populate the credential cache…

4. (OPTION 2) Use az CLI to store your preferred account into the credential cache

   This is only required if your preferred credential is not your main Visual Studio account.
   
   To perform this step, you’ll need the az CLI – which you can download from here:
   https://docs.microsoft.com/en-us/cli/azure/install-azure-cli
   
   Then, open a command line and run “az login” with your user.